JERUSALEM -- At least nine pro-Palestinian activists were killed and dozens were wounded aboard an aid flotilla bound for the Gaza Strip when Israeli naval commandos seized control of the boats in international waters early Monday.

An Israeli army spokesman would not disclose their identities or nationalities. Some Turkish, Israeli and Arab media outlets had earlier put the death toll at 19 activists and said they included as many as 10 Turkish nationals. The wounded were evacuated to Israeli hospitals and the ships were led into Israel's Ashdod port, where the passengers and aid supplies were being unloaded and screened. Seven Israeli naval personnel were also injured.

Sharp condemnations of Israel rang out from across the world, with several European countries summoning Israeli ambassadors to protest. The European Union called for an inquiry into the deaths. And the United Nations Security Council planned to meet Monday afternoon for an emergency session. The United States expressed regret at the loss of the life and said it was "working to understand the circumstances of the tragedy."

Israeli Prime Minister Binyamin Netanyahu, who is in Canada and had been scheduled to meet President Obama in Washington on Tuesday, decided to cut short his trip and will return home Monday night instead, his office said.

Turkey, which dispatched the main Mavi Marmara ship carrying 600 activists and thousands of tons of aid, strongly condemned Israel, warning of deep consequences to relations, as protesters demonstrated outside the Israeli consulate in Istanbul and Israel released a travel advisory warning Israelis to avoid travel to Turkey.

"We strongly condemn these inhumane acts of Israel. This grave incident which took place in high seas in gross violation of international law might cause irreversible consequences in our relations," the Turkish Foreign Ministry said in a statement.

Israel had warned the organizers of the flotilla -- transporting items such as concrete to help Gaza rebuild after last year's war with Israel -- that they would not be allowed to sail directly through the blockaded region.

The activists refused to dock in Israel and have relief supplies unloaded and inspected there, saying they did not trust that Israel would allow the contents to be trucked to the Gaza Strip.

Overnight, Israeli naval personnel dropped from helicopters onto the largest passenger ship from Turkey, which had mostly Turkish nationals aboard. Short video clips broadcast on television stations showed demonstrators clubbing the navy personnel with metal bars and showed at least one Israeli commando firing.

Israeli military officials said demonstrators attacked the navy personnel with knives and live fire and seized at least one of the soldiers' weapons.

"This IDF naval operation was carried out under orders from the political leadership to halt the flotilla from reaching the Gaza Strip and breaching the naval blockade," the Israeli army said in its statement.

Past flotillas either reached Gaza or were diverted to Israel peacefully. But the activists, frustrated with a continuing blockade of Gaza, mounted a much larger effort this time to challenge the siege. Israel, amid growing tensions with Turkey and anxieties about the Turkish organizers' contacts with what Israeli officials described as terrorist groups, said it needed to keep the siege in tact.

Some in Israel, before the raid and after, questioned the wisdom of Israel trying to take the ships by force.

Israel has managed a blockade on Gaza in an effort to isolate the Islamist leadership of Hamas, which took control of the strip of territory in 2007. Israel allows goods to pass into the Gaza Strip but limits, or prevents, certain categories needed especially for construction. Many goods are smuggled into Gaza through underground tunnels from Egypt, creating an overall dysfunctional economy reliant on the decisions of outside powers.

On Monday, after the clash at sea, Israel closed one of the main border crossings into Gaza and turned back roughly 60 trucks of goods destined for the strip.

Flotilla organizers, from the Turkish non-governmental organization IHH, or Humanitarian Relief Fund, said they were transporting 6,000 tons of cement, more them 2,000 tons of iron, 100 prefabricated houses, 500 wheelchairs, medical equipment, wood and glass for building, as well as electric generators and food.

Reports of the activists' deaths prompted demonstrations in the Hamas-ruled Gaza Strip, and Israel increased the presence of police and military across the country in anticipation of possible riots. Palestinian Authority President Mahmoud Abbas declared three days of mourning.

A committee representing the Arab Israeli community declared a general strike for Tuesday. Already on Monday, Arab merchants in the Old City of Jerusalem shuttered their shops and restaurants in protest.

"We are shocked by reports of killings and injuries of people on board boats carrying supplies for Gaza, apparently in international waters," United Nations Special Coordinator for the Middle East Peace Process Robert Serry and Filippo Grandi, commissioner-general of the U.N. Relief and Works Agency, said in a statement. "We condemn the violence and call for it to stop," they said, adding that they were seeking a "full explanation" from Israel.

"We wish to make clear that such tragedies are entirely avoidable if Israel heeds the repeated calls of the international community to end its counterproductive and unacceptable blockade of Gaza," the statement by the U.N. officials said.